From mboxrd@z Thu Jan 1 00:00:00 1970 From: Earl Chew Subject: Re: [PATCH 1/1] Fix seq_file mishandling of consecutive pread() invocations. Date: Sun, 22 Jan 2012 11:01:21 -0800 Message-ID: <4F1C5D01.3060403@ixiacom.com> References: <4F166FC9.8030906@ixiacom.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: "linux-kernel@vger.kernel.org" , , To: Alexander Viro Return-path: Received: from am1ehsobe002.messaging.microsoft.com ([213.199.154.205]:42580 "EHLO AM1EHSOBE002.bigfish.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752057Ab2AVTBd (ORCPT ); Sun, 22 Jan 2012 14:01:33 -0500 In-Reply-To: <4F166FC9.8030906@ixiacom.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: [ Added Maintainers; Added reference to bugzilla.kernel.org in commit log ] Also reported in: https://bugzilla.kernel.org/show_bug.cgi?id=11856 The following program illustrates the problem: char buf[8192]; int fd = open("/proc/self/maps", O_RDONLY); n = pread(fd, buf, sizeof(buf), 0); printf("%d\n", n); /* lseek(fd, 0, SEEK_CUR); */ /* Uncomment to work around */ n = pread(fd, buf, sizeof(buf), 0); printf("%d\n", n); The second printf() prints zero, but uncommenting the lseek() corrects its behaviour. To fix, make seq_read() mirror seq_lseek() when processing changes in *ppos. Restore m->version first, then if required traverse and update read_pos on success. Signed-off-by: Earl Chew --- fs/seq_file.c | 28 +++++++++++++++------------- 1 files changed, 15 insertions(+), 13 deletions(-) diff --git a/fs/seq_file.c b/fs/seq_file.c index dba43c3..7a45306 100644 --- a/fs/seq_file.c +++ b/fs/seq_file.c @@ -140,9 +140,21 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos) mutex_lock(&m->lock); + /* + * seq_file->op->..m_start/m_stop/m_next may do special actions + * or optimisations based on the file->f_version, so we want to + * pass the file->f_version to those methods. + * + * seq_file->version is just copy of f_version, and seq_file + * methods can treat it simply as file version. + * It is copied in first and copied out after all operations. + * It is convenient to have it as part of structure to avoid the + * need of passing another argument to all the seq_file methods. + */ + m->version = file->f_version; + /* Don't assume *ppos is where we left it */ if (unlikely(*ppos != m->read_pos)) { - m->read_pos = *ppos; while ((err = traverse(m, *ppos)) == -EAGAIN) ; if (err) { @@ -152,21 +164,11 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos) m->index = 0; m->count = 0; goto Done; + } else { + m->read_pos = *ppos; } } - /* - * seq_file->op->..m_start/m_stop/m_next may do special actions - * or optimisations based on the file->f_version, so we want to - * pass the file->f_version to those methods. - * - * seq_file->version is just copy of f_version, and seq_file - * methods can treat it simply as file version. - * It is copied in first and copied out after all operations. - * It is convenient to have it as part of structure to avoid the - * need of passing another argument to all the seq_file methods. - */ - m->version = file->f_version; /* grab buffer if we didn't have one */ if (!m->buf) { m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL); -- 1.7.0.4